Variable transmission line



Nov. 7, 1961 c. B. WATTS, JR 3,008,101

VARIABLE TRANSMISSION LINE Filed May 1, 1959 RF DELAY INVENTOR. l6

6265391 3 WZZZ J1:

COIL CUR/PEN T A TTORNEYS United States Patent 3,008,101 VARIABLE TRANSMISSION LINE Chester B. Watts, Jr., Alexandria, Va., assignor to ScanwellLaboratones, Inc., Fairfax, Va., a corporation of Virginia Filed May 1, 1959, Ser. No. 810,336 4 Claims. (Cl. 33331) This invention relates to the transmission of radiofrequency or microwave energy, and more specifically to the rapid control of the phase or the amplitude of the voltage and current associated with such energy.

It IS often desirable, in radio-frequency systems, to be able, in response to a control signal, to change the RF phase of a signal in a transmission line without at the same time causing appreciable amplitude change. This has been done in a number of different ways by devices known as variable delay lines or phase-shifters. It is ordinarily possible to classify such devices either as electro-mechanical or as non-mechanical in nature. The electro-rnechanical types are characterized by relatively high stability and goor linearity but usually respond rather slowly to a control signal.

It is an object of this invention to provide an electromechanical type of phase-shifter wherein the required mechanical motion is small and the mass of the moving part is also small, thereby permitting relatively rapid response to a control signal.

Another object of this invention is to provide an adjustable low-pass wave filter wherein the required mechanical motion is-small and the mass of the moving part is also small, thereby permitting rapid variation of the cutoff frequency in response to a control signal.

An embodiment of this invention provides a coaxial transmission line type of delay structure wherein the inner and the outer conductors carry matching sets of radial fin-like projections. The phase-velocity of the delay structure changes from a maximum value to a minimum value when the inner conductor is moved longitudinally the small distance corresponding to one-half of the distance between projections. The motion of the inner conductor is produced by a driving coil in a permanent magnet field after the fashion of an ordinary dynamic loudspeaker.

FIG. 1 is a drawing of a complete device which embodies this invention;

'FIG. 2 is a sectional view of a portion of a coaxial transmission line delay structure set for minimum delay, or maximum phase velocity;

FIG. 3 is a sectional view of the coaxial transmission line delay structure with inner conductor shown in position for maximum delay, or minimum phase-velocity;

FIG. 4 is an approximate equivalent electrical circuit for the variable transmission line delay structure; and

FIG. 5 is a graph showing the manner in which the radio-frequency delay varies with current applied to the driving coil.

The device may be described in more detail as follows: In FIG. 2 a sectional view of a portion of the coaxial transmission line delay structure is shown. The outer conductor 1 carries a set of annular fin-like projections 2 which extend toward the inner conductor 3. The inner conductor 3 carries a corresponding set of annular finlike projections 4 which extend outwardly. FIG. 2 shows the inner conductor 3 positioned longitudinally so as to place the inner conductor projections 4 axially mid-way between the outer conductor projections 2. This is called the minimum-delay position. In FIG. 3, the same parts are shown, but the inner conductor 3 is axially positioned so as to place the inner conductor projections 4 directly opposite the outer conductor projections 2. This is called the maximum-delay position.

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The electrical properties of a transmission line may be approximately described by the equivalent circuit of FIG. 4 for frequencies that are low enough to keep the phaseshift per section small. As is well known, the line then has a characteristic impedance Z given by and also has a time delay per section given by T= /LC where the parameters L and C are the inductance and capacitance per section.

In FIG. 4, the parameters L and C are to be considered both as variables in the present invention, by virtue of the longitudinal motion of the inner conductor 3 of the transmission line delay structure. However, it is considered preferable that the structure be so proportioned that L and C vary together so that their ratio remains nearly constant. This tends to produce a line of constant characteristic impedance whose delay varies in accordance with the square root of the product LC.

In FIG. 2, the LC product is at a minimum, but it is somewhat larger per unit length than for a straight coaxial line without projecting fins. This may be thought of as being due to the meandering of the equipotential lines, typified by line 5 and to the concentration of electric flux lines at the corners of the projections 2 and 4, typified by line 6. However, when the inner conductor 3 moves to the position shown in FIG. 3, there is first a greater concentration of electric flux lines between the projections 2 and 4, and second, more spreading apart of the equipotential lines 5 in the spaces between adjacent pairs of projections. The former may be considered as an increase in capacitance C in the regions typically labelled 7 in FIG. 3, while the latter may be considered as an increase in inductance L in the regions typically labelled 8 in FIG. 3.

A complete device which embodies this invention is shown in FIG. 1. The parts 1, 2, 3, and 4 of the delay structure are again shown. It is understood that the delay structure may have as many sections as are necessary to achieve the desired total delay variation. The inner conductor 3 receives mechanical support for axial movement from a flexible metal diaphragm 9 at one end and from another similar diaphragm, not shown, at the other end. These diaphragms, while providing transverse support, allow limited longitudinal motion of the inner conductor 3. Diaphragm 9 is electrically insulated from inner conductor 3 by means of insulating bead 10. Connectors 11 and 12 are provided for conveying radio-frequency signals into and out of the device. The connection to inner conductor 3 from connector 12 is made through flexible lead 13, and terminal member 20, insulated from outer conductor 1 by insulating bushing 22. Similar structure is provided at connector 11 to provide an insulated lead to the other end of conductor 3.

Driving coil 14 is in the magnetic field of permanent magnet 15. Current in coil 14 results in force applied through insulating thrust rod 16, thereby deflecting the diaphragms and moving inner conductor 3 some distance longitudinally. The distance rnoved is approximately proportional to the current in coil 14.

The manner in which the radio-frequency delay varies with coil current is shown typically in FIG. 5. in this case, the initial or neutral position of the inner conductor 3 is chosen to be half way between the minimum delay point 16' and the maximum delay point 17. This choice makes use of both positive and negative driving coil current. There is in general a portion of the delay curve between points 18 and 19 which is essentially straight. This portion of the curve is utilized when it is desired to have the device introduce delay or phase shift which is proportional to the control current.

There is an alternative mode of operation in which the device functions as an adjustable low-pass filter. Referring again to FIG. 2, it can be understood that if the frequency of the applied radio-frequency signal is sufficiently increased, then no longer can the phase shift per section be considered small, and eventually a frequency is reached where the signal is not passed through the structure. This cut-off frequency i is known by conventional filter theory to be given by the formula The product LC is variable by the control means already described. This implies that the device may be used as an amplitude modulator or switch by varying the cut-off frequency from a point above to a point below a chosen band of signal frequencies.

While a single specific embodiment of the invention has been shown and described herein, it is to be understood that other forms may be resorted to within the scope of the appended claims.

I claim:

1. A variable radio frequency device comprising, a coaxial transmission line having an outer conductor, an inner conductor extending axially and concentrically of said outer conductor, means mounting said inner conductor for free axial reciprocation within said outer conductor, said outer conductor having a series of longitudinally spaced annular ribs extending inwardly, said inner conductor having a series of annular ribs extending outward- 1y therefrom and being longitudinally spaced correspondingly to the spacing between said ribs on said outer conduetor, and driving means for axially reciprocating said inner conductor said driving means comprising a stationary magnet and a moveable coil in the field of said magnet, said coil being drivingly connected to said inner conductor.

2. A device as defined in claim 1 including resilient means normally holding said inner conductor and coil in a predetermined axial position, whereby the distance the inner conductor moves axially is substantially proportional to the current flowing in said coil.

3. A device as defined in claim 2 wherein said resilient means also constitutes said means for mounting said inner conductor for axial reciprocation.

4. A device as defined in claim 2 wherein, when said inner conductor is in said predetermined axial position, the adjacent peripheral surfaces of said ribs are in only partially overlapping relation.

References Cited in the file of this patent UNITED STATES PATENTS 2,375,004 Knowles May 1, 1945 2,446,835 [Keary Aug. 10, 1948 2,527,608 Willoughby Oct. 31, 1950 2,604,594 'White July 22, 1952 2,919,418 Reis et al. Dec. 29, 1959 FOREIGN PATENTS 934,354 Germany Oct. 20, 1955 

